The significance of acoustics in the design of aero gas turbine engines has increased in recent years as a consequence of stringent requirements imposed by regulatory authorities on the noise radiated during particularly the take-off and approach phases of flight.
The noise produced by an aero gas turbine engine is the accumulation of many individual noise sources. Sources related to the turbomachinery (fan, turbine, compressor) contain both broadband and tonal components, while the jet noise source is broadband in nature.
The fan is typically a major source of tone noise emanating from the inlet, particularly when the engine is at an operating condition which generates supersonic fluid flow at the fan blades. In this case, tone noise is produced by shock waves emanating from the fan blades and propagating upstream through the engine intake.
Such tone noise typically has components at the fan blade passing frequency (BPF) and at frequency multiples thereof. The noise can also include “buzz” caused by variations in the shock waves produced by adjacent fan blades. The variations themselves can be caused, for example, by small differences in manufactured fan blade shapes. Buzz occurs at harmonics of shaft rotation frequency. Although buzz can be the predominant part of the tone noise produced by a fan, it is effectively a distortion of the BPF tone and its harmonics. Therefore, if the BPF tone and its harmonics are attenuated close to the source, buzz should also be attenuated.
An important way of reducing noise levels is to line the intake and bypass ducts with panels that absorb the sound produced by the fan system.
It has also been suggested to counteract the tone noise produced by a fan by matching the fan blades with axially spaced stator vanes. For example, U.S. Pat. No. 5,169,288 proposes a fan assembly in which the number of stator blades is selected to obtain equal values of a cut-off ratio for at least two BPF harmonics.
J. M. Tyler and T. G. Sofrin, “Axial Flow Compressor Noise Studies”, SAE Transactions, Vol. 70, 1962, pp. 309-332 provides a “spinning mode theory” which has been applied by other workers to understand and combat tone noise produced by aero gas turbine engines.
D. Prasad and J. Feng, “Propagation and Decay of Shock Waves in Turbofan Engine Inlets”, Proceedings of ASME Turbo Expo 2004, Jun. 14-17, 2004, Vienna, Austria describes numerical experiments carried out to investigate the tone noise radiated from a turbofan engine inlet at supersonic operating conditions. The experiments focused on the attenuation of acoustic power produced by changes to the shape of the engine nacelle.
Of course, the final design of aero engines, and particularly fan blades, is influenced by a number of factors, of which engine noise reduction may be but one. Thus, U.S. Pat. No. 6,071,077 describes a swept fan blade which aims to increase fan blade efficiency and resistance to foreign object damage. The blade has, among other characteristics, a leading edge of variable sweep angle which, with increasing radial distance, changes from forward swept to rearward swept to forward swept again. U.S. Pat. No. 6,358,003 describes another swept fan blade which aims to increase fan blade efficiency and flow stability. This blade has, in the tip region, a leading edge with forward-backward sweep.